The invention relates generally to blood pressure measurement. More particularly, the invention relates to a method and system for simultaneous, bilateral, segmental measurement of blood pressure using Doppler techniques.
Segmental blood pressure is one of the most important parameters to measure in understanding and diagnosing vascular diseases. Blood pressure measurement (sphygmomanometry) for segmental systolic pressures using Doppler techniques has gained wide acceptance, and is the modality of choice in many incidences. This is in large part because of Doppler""s greater sensitivity and accuracy when compared with photoplethysmographic and oscillometric methods and the Doppler method""s lesser sensitivity to temperature fluctuations. The Doppler-based systems also have the desirable characteristic of being vessel-specific.
One application of Doppler sphygmomanometry is in segmental blood pressure measurements, i.e., tracking blood pressures at different parts of the body. For example, because of its vessel-specificity, Doppler sphygmomanometry can be used to trace blood pressures along an artery in limbs for diagnosis of peripheral arterial disease (PAD).
In performing segmented blood pressure measurements, it is often desirable to conduct bilateral measurements, i.e., measurements at symmetrically paired locations, such as left and right feet, and left and right forearms. The comparison between blood pressures at symmetrical locations may yield clues to the condition of blood vessels on either side. The ideal time for making such comparisons would be when the blood pressures at both locations are measured for the same heartbeat so that any differences between the measurements are not attributable to the difference in the different heartbeats.
However, simultaneous bilateral Doppler measurements have not been successfully used in conventional systems and techniques. One conventional approach is to employ two separate Doppler circuits operating at nominally the same frequency, e.g., 8 MHz. Such approach suffers from at least two drawbacks. First, it is difficult to maintain an exact frequency match between the two circuits; second, as schematically illustrated in FIG. 9, even if the drivers 910, 920 of the two Doppler circuits could be matched exactly, or even if the two drivers were in fact a common driver, the signals in both circuits are like to cross-couple (930) with each other, both electronically and acoustically due to the nearly-identical frequencies of the signals. The magnitude of the error signals resulting from such interference is in the same bandwidth, and can be on the same order of magnitude or even greater than, the useful signals. Driving two transducers from a common signal driver to eliminate frequency drift does not solve the cross coupling problem as the return Doppler signals can cross couple in the same way as heretofore described.
The invention disclosed herein is aimed at providing a system and method for simultaneous, bilateral measurement of segmental blood pressures substantially without the drawbacks of the conventional approaches.
Generally, the invention provides a system and method whereby two Doppler measurements of blood flow at two different locations on a body are carried out substantially simultaneously, i.e., within the same heartbeat with minimum interference from each other.
In one illustrative embodiment of the invention, two Doppler modules are configured to operate at nominally the same Doppler carrier frequency so that they probe for substantially the same range of physiological information. However, the Doppler carrier frequencies for the Doppler modules are also sufficiently different to substantially avoid interference between the two modules. A phase-locked loop (PLL) used to demodulate the velocity signals in each Doppler module efficiently blocks interference signals because they have frequencies that are the difference in the Doppler carrier frequencies and that are outside the locking range of the PLL. Broad-band RF amplifiers and Doppler transducers are used to ensure that the frequency responses of the Doppler modules are substantially the same.